This invention relates to a telecommunication system, particularly, but not exclusively, applicable to links between a satellite and land-mobile terminals. More particularly, the invention relates to a method of and apparatus for data transmission which enable detection and correction of errors introduced during transmission and utilize a new ARQ principle (Automatic Retransmission on request).
The essential objective of a data transmission method of this kind is to transform and encode a data message into another message less vulnerable to transmission errors and equivalent to the initial message. It is known to attain this objective by using a redundant code which enables the initial message to be reconstituted from reception of a coded message which is incomplete or includes errors.
In the case of a satellite to-earth station link, for which the invention is particularly suitable, the transmission of data is frequently affected by periods of fading or even complete interruption of communication (black-out). FIGS. 3 and 4 of the accompanying drawings, which represent the received signal displayed on the cathode screen of a spectrum analyzer, show typical examples of such disturbances: the signal of FIG. 3 corresponds to periodic fading such as may be produced by regularly spaced trees along a road along which a receiver vehicle is driving; FIG. 4 corresponds to temporary fading caused by passing under a bridge which crosses a motorway.
However, most of the time, reception is good, for example when the vehicle is on a main road in the country-side. In this case, the link is identical to a white noise Gaussian channel.
The invention aims to provide a data transmission system for use between satellites and mobile terminals for example, which has high transmission throughput, while limiting the risk of producing procedural ambiguities during poor propagation conditions.
Various transmission systems known to solve certain problems due to poor propagation conditions are of the hybryd ARQ type; that is to say, they are based on automatic retransmission on request when data has been lost. The name of these systems derives from the RQ symbol, for retransmission request, in the telegraph system patented in 1943. The transmission system is called hybrid when it also uses in parallel an error correction process.